1. Field of the Invention
This invention relates to grinding abrasive grains suitable for grinding semiconductor wafers such as compound semiconductor wafers (e.g., GaAs wafers), silicon wafers, etc., an abrasive containing the abrasive grains, an abrasive solution containing the abrasive grains, a method for preparing the abrasive solution, a grinding method by the abrasive solution, and a semiconductor device fabrication method using the abrasive solution.
2. Description of the Related Art
In recent years, the flatness of compound semiconductor wafers or semiconductor wafers has been strictly required with remarkable high integration and high capacity, so that still more excellent-precision machining technology has been required. In the grinding step of a wafer fabrication process, there are many problems of removing shape irregularities, removing thickness variations, removing machining strains, adjusting flatness, etc.
In general, combined chemical and mechanical grinding is applied to the grinding step, and abrasive is a consumable item which largely affects wafer quality. In the chemical and mechanical grinding of semiconductor wafers such as compound semiconductor wafers, silicon wafers, etc., it is important to balance a chemical factor which serves to reduce machining strains and a mechanical factor which affects flatness and surface quality. Both chemical and mechanical factors are largely related to grinding characteristics of abrasive.
In other words, the grinding step of semiconductor wafers such as compound semiconductor wafers, silicon wafers, etc., comprises the primary step of forming an oxide film on a surface, the secondary step of grinding the oxide film by abrasive grains, and the third step of dissolving the ground oxide to increase grinding speed. The primary and third steps affect a chemical factor, and the secondary step affects a mechanical factor. Of these steps, the secondary step of grinding the oxide film by abrasive grains largely affects a wafer surface state, shape and flatness, and determines wafer quality. Abrasive grain form affects all grinding characteristics including grinding speed.
Accordingly, the most important factor in abrasive that affects wafer quality is abrasive grains. Abrasive grains are largely divided into abrasive grains which disperse in water as primary grains, and abrasive grains which disperse as secondary grains consisting of a plurality of aggregated primary grains. Which of the primary and secondary grains to use, or what ratio to mix them depends on objects to be ground or machining purposes.
On the other hand, for the purpose of enhancement in grinding characteristics, various suggestions which focus on abrasive grain shapes have been made by each material maker.
Japanese patent application laid-open No. 7-221059 describes that, in grinding a semiconductor wafer, grinding speed is faster by grinding a colloidal silica shape observed by an electron microscope with non-spherical abrasive grains defined by minor to major axes ratios. As a result of examination on this by the present inventor, the grinding speed of amorphous abrasive grains typically tends to be faster than that of spherical abrasive grains. Even though abrasive grains are amorphous, however, its grinding speed is not necessarily faster. Also, when abrasive grains are observed with an electron microscope, unlike a dispersed state in an actual solution, tertiary grains consisting of a plurality of aggregated secondary grains or quaternary grains consisting of a plurality of aggregated tertiary grains are observed, which causes many errors, and are not efficient.
Japanese patent application laid-open No. 2001-11433 describes that the grinding speed of abrasive grains consisting of spherical colloidal silica grains linked within one plane by adding a divalent or trivalent metal oxide to the spherical colloidal silica grains is faster, and that a flat smooth surface is obtained with high precision. In this case, however, added metal ions are an impurity which adversely affects a wafer surface.
Japanese patent application laid-open No. 9-296161 discloses an abrasive using abrasive grains for reducing defects (e.g., flaws), in which water is caused to intervene between primary grains to form elastic secondary grains by heating a high-purity fumed silica solution. However, control of grain size and secondary grain shape is difficult, and stable grinding characteristics cannot be obtained, which results in no effect on some objects to be ground.
Japanese patent application laid-open No. 2002-338232 discloses an abrasive composition using aggregated spherical colloidal silica grains with its uneven surface as abrasive grains, aggregated by a flocculant and aggregation aid. It describes use of polyaluminum chloride as the flocculent, however, such added metals are a kind of impurities, which results in a contaminated wafer surface to be ground.
Compound semiconductor wafers and semiconductor wafers are fabricated by means of primary grinding (rough grinding) and secondary grinding (mirror finish). Typically, the primary grinding is performed by pressing a wafer on a soft abrasive pad of a nonwoven cloth or a suede type, and exerting a constant pressure, dripping an abrasive consisting of abrasive grains, oxidizer, alkali reagent, etc. Large problems with the primary grinding step lie in ensuring a wafer shape is of a stable precision, and realizing high-speed grinding. In particular, wafer shape is important, where failure to ensure a wafer shape with a stable precision would affect the subsequent secondary grinding step. The secondary grinding step is the mirror finish step by smoothing wafer surface roughness, often using an abrasive without abrasive grains, which results in no shape correction capability. In other words, it is necessary to constantly ensure a wafer shape with a stable precision in the primary grinding step. In particular, a soft abrasive pad of a nonwoven cloth or a suede type typically tends to cause peripheral dripping in the wafer, compared to a abrasive pad made of foamed polyurethane used in CMP (Chemical Mechanical Polishing), which results in difficulty in obtaining good flatness.
Also, there is the large problem that the above soft abrasive pad degrades fast, grinding characteristics changing with the degradation of the abrasive pad, which results in more remarkable peripheral dripping in a wafer peripheral portion. As a method for improving malfunction caused by the abrasive pad, there is the method for improving flatness by increasing the elasticity of the abrasive pad, but using this would rather increase the incidence of flaws.
On the other hand, in the research and development of abrasives, as described in the above patent references, various researches have been made on grinding speed, surface roughness, and the reduction of defects (e.g., flaws), but as it stands, there are no disclosures about techniques for enhancing flatness by preventing peripheral dripping, or enhancing wafer shape stability over time.